Waterborne nitrocellulose/acrylic lacquer emulsions were previously known from U.S. Pat. No. 3,953,386 and 4,011,388. While these emulsions were fully functional, they failed to gain acceptance in the furniture finishing trade, where solvent based lacquers continue to be used almost uniformly.
Vanderhoff et al., U.S. Pat. No. 4,177,177, outlines three emulsification methods for making latices such as a waterborne nitrocellulose/acrylic emulsion. These are sometimes referred to as "artificial latices".
(1) Emulsification of the Polymer Solution. In this method the polymer is dissolved in a volatile solvent or a mixture of solvents which are immiscible with water. The polymer solution is then dispersed, usually by some kind of mechanical means, in water in the presence of surfactants. The volatile solvent is then removed by vacuum distillation followed by concentrating the resulting dilute latex to the desired solids. This method is reported to give latices with particles of about 1 micron in diameter.
(2) Emulsification by Phase Inversion. In this method the polymer which is being emulsified is mixed with surfactant and a volatile solvent or a mixture of solvents for the polymer. Then water is added to this solution. In the early stages of water addition, a water-in-oil emulsion is formed which upon further addition of water inverts to an oil-in-water emulsion. According to the Vanderhoff et al. patent, this method calls for greater care and control than method (1) and in addition yields latex particle diameter of about 0.8-1.0 micron or larger.
(3) Self emulsification. This method requires polymer molecules to be chemically modified by incorporation of functional groups in such a way that the polymer becomes self emulsifiable in aqueous media. This method is reported to Yield very small size (0.1 u) particles. The main disadvantage of this method is reported to be the water-sensitivity of the film due to significant proportions of the functional groups of the polymer backbone.
Spontaneous emulsification is also described in the literature as another method of making artificial latices. This method, similar to the self emulsification method described above, however, does not require the chemical modification of the polymer molecules.
Spontaneous emulsification is reported to be the result of interfacial turbulence, diffusion of one of the components from one phase to the other, and by negative interfacial tension between two phases from P. Z. Becher, Encyclopedia of Emulsion Technology Vol. 2, p. 281.
The Vanderhoff et al. patent concludes that the first method is a preferred way of making artificial latices. Particle size is the only disadvantage they see in the process. This patent teaches the method to reduce the particle size to the desired range by using mixed emulsifiers (emulsifier + long chain alkane or long chain alcohols). Using this technology, they reported to make latices with particle diameters of &lt;0.5 micron.
Thus, it remained for the present invention to provide a sufficient degree of improvement in waterborne nitrocellulose/acrylic emulsions that advantages would outweigh disadvantages in order to induce a switch from tried and true techniques of solvent application.